Structure and bonding in the gaseous monochlorides, trichlorides and trimethyl derivatives of the Group 13 elements
The Group 13 elements from B to Tl form mono- and tri-halides. All monomeric trihalides are trigonal planar in the gas phase. The stability of a given trichloride (MCl3) relative to the monochloride (MCl) decreases as the group is descended. Except for M = B, the bonds in the trichloride are shorter but weaker than in the monochloride. Calculations based on the spherical ion model yields M-Cl dissociation energies of the monomeric monochlorides, which are at least 30% smaller than the experimental value. Similar calculations on the trichlorides yield mean M-Cl bond energies which are at least 80% smaller than the experimental. The bonding is therefore described here as polar covalent. The bonding in the trichlorides are discussed in terms of sp 2 hybridization and two-center molecular orbitals. The structures, bond distances and mean M-C bond energies of the monomeric trimethyl derivatives M(CH3)3 are described.
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